Although they does have proper abuse policies and do take action against spammers. I don't get any spam from them (except perhaps the very occasional one), and I know businesses that use mailchimp and similar services for valid marketing (to previous customers). Just looking through my received mailbox, I see many legitimate emails from mailchimp.

I'm not denying that they are sometimes used by spammers, but they are definitely a legitimate operation that takes action against spammers if you report them.